In recent years, steels for welded structures have been required to have low yield strength and high uniform elongation in addition to high strength and high toughness from an earthquake-proof point of view. In general, it is known that steel is enabled to have low yield strength and high uniform elongation in such a manner that the metallographic microstructure of the steel is transformed into a microstructure in which a hard phase such as bainite or martensite is adequately dispersed in ferrite, which is a soft phase. The term “uniform elongation” as used herein is also called even elongation and refers to the limit of the permanent elongation of a parallel portion of a specimen uniformly deformed in a tensile test. The uniform elongation is usually determined in the form of the permanent elongation corresponding to the maximum tensile load.
As for manufacturing methods capable of obtaining a microstructure in which a hard phase is adequately dispersed in a soft phase as described above, Patent Literature 1 discloses a heat treatment method in which quenching (Q′) from the two-phase, (γ+α) temperature range of ferrite and austenite is performed between quenching (Q) and tempering (T).
As for methods in which the number of manufacturing steps is not increased, Patent Literature 2 discloses a method in which after rolling is finished at the Ar3 transformation temperature or higher, the start of accelerated cooling is delayed until the temperature of a steel material decreases to or below the Ar3 transformation temperature, at which ferrite is produced.
As for techniques for achieving low yield ratio without performing such heat treatment as disclosed in Patent Literature 1 or 2, Patent Literature 3 discloses a method in which low yield ratio is achieved in such a manner that after the rolling of a steel material is finished at the Ar3 transformation temperature or higher, the rate of accelerated cooling and the finishing cooling temperature are controlled such that a two-phase microstructure consisting of acicular ferrite and martensite is produced.
Furthermore, as for techniques for achieving low yield ratio and excellent welded heat affected zone (HAZ) toughness, Patent Literature 4 discloses a method in which a three-phase microstructure consisting of ferrite, bainite, and Martensite-Austenite constituent (island martensite, Martensitic Islands or M-A constituent, hereinafter called M-A constituent) is produced in such a manner that Ti/N and/or the Ca—O—S balance is controlled.
Patent Literature 5 discloses a technique in which low yield ratio and high uniform elongation are achieved by the addition of an alloying element such as Cu, Ni, or Mo.
On the other hand, welded steel pipes such as UOE steel pipes used for line pipes and electric welded tubes are manufactured in such a manner that steel plates are cold-formed into pipes, abutting surfaces thereof are welded, and the outer surfaces of the tubes are usually subjected to coating such as polyethylene coating or powder epoxy coating in view of corrosion resistance. Therefore, there is a problem in that the steel pipes have a yield ratio greater than the yield ratio of the steel plates because strain ageing is caused by the strain during pipe making and the heat during coating and the yield stress is increased. In order to cope with such a problem, Patent Literatures 6 and 7 each disclose a steel pipe which has excellent strain ageing resistance, low yield ratio, high strength, and high toughness and which makes use of fine precipitates of composite carbides containing Ti and Mo or fine precipitates of composite carbides containing two or more of Ti, Nb, and V and also disclose a method for manufacturing the steelpipe.